Two waves of intensity ratio $1: 9$ cross each other at a point. The resultant intensities at that point, when (a) Waves are incoherent is $I_1$ (b) Waves are coherent is $I_2$ and differ in phase by $60^{\circ}$. If $\frac{I_1}{I_2}=\frac{10}{x}$ then $x=$ _________.